* syscall.c (cb_syscall) <case CB_SYS_pipe>: New case.

	* syscall.c (cb_syscall) <case CB_SYS_pipe>: New case.
	* callback.c [HAVE_LIMITS_H]: Include limits.h.
  	Include libiberty.h.
	(os_close, os_read, os_write, os_fstat, os_ftruncate): Support fd
	being either end of a pipe.
	(os_pipe, os_pipe_empty, os_pipe_nonempty): New functions.
	(os_shutdown): Clear pipe state.
	(default_callback): Initialize new members.